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Intangible assets - Summary of Amortization expense (Detail) - EUR (€)
€ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of detailed information about intangible assets [line items]      
Amortisation, intangible assets other than goodwill € 126 € 111 € 118
Research and development expenses [Member]      
Disclosure of detailed information about intangible assets [line items]      
Amortisation, intangible assets other than goodwill 31 28 9
General and administrative expenses [Member]      
Disclosure of detailed information about intangible assets [line items]      
Amortisation, intangible assets other than goodwill € 95 € 83 € 109